TECTONOSTRATIGRAPHIC EVOLUTION AND HYDROCARBON PROSPECTIVITY SOUTH OF GWADAR BAY, MAKRAN ACCRETIONARY WEDGE, OFFSHORE SW PAKISTAN

The Makran accretionary wedge developed as a result of subduction of the Arabian Plate beneath the southern margin of Eurasia since the Eocene. Interpretation of 2D seismic profiles calibrated to offshore well data in a study area to the south of Gwadar Bay (SW Pakistan) indicates a major period of accretion from the mid-Miocene, as evidenced by the occurrence of thick growth strata associated with large-scale imbricate thrusts. The thrust faults originate from a deep detachment within the mud-rich Oligocene interval, and well-developed piggy-back basin successions occur in thrust hanging walls. In the study area, the thrust structures are sealed by a thick, progradational Pliocene to Recent interval in which the presence of submarine canyons, up to 2.5 km across, indicate that sedimentary transport was from the north.

Fluid escape pipes and associated amplitude anomalies are observed in the seismic profiles studied and may be related to upward migration of thermogenic hydrocarbons from depth, as heavy hydrocarbon fractions <C₃ have been reported from nearby wells. The hydrocarbons are believed to have been sourced from the Oligocene Hoshab Shale and to have then migrated up through a sedimentary succession in which permeability barriers are largely absent. Hanging wall anticlines mapped in the study area could provide structural traps, and turbidites in the Lower Miocene Panjgur Formation may represent a potential reservoir. Amplitude anomalies are also observed adjacent to shallow fluid escape pipes within the topsets of clinoforms in the Pleistocene Chatti and Omara Formations, and probably indicate the presence of biogenic hydrocarbons sourced from distal mudstones in bottomset strata.